Comparison of two methods of quantitation in human studies of biodistribution and radiation dosimetry

A simple method of quantitating organ radioactivity content for dosimetry purposes, based on relationships between organ count rate and the initial whole body count rate, has been compared with a more rigorous method of absolute quantitation using a transmission scanning technique. Comparisons were on the basis of organ uptake (% administered activity) and resultant organ radiation doses (mGy MBq-1) in 6 normal male volunteers given a 99Tcm-labelled myocardial perfusion imaging agent intravenously at rest and following exercise. In these studies, estimates of individual organ uptakes by the simple method were in error by between +24 and -16% compared with the more accurate method. However, errors on organ dose values were somewhat less and the effective dose was correct to within 3%. These radiation dose effects were largely influenced by radioactivity in the remaining body (i.e. excluding the organs specifically analysed) which accounted for the major proportion of the 99Tcm residence time. Effects due to different biodistributions between rest and exercise studies were also observed and it is concluded that although the simple method was acceptable for the present dosimetry study it should only be used with caution, and a more accurate method is recommended.